Organic electronic devices that emit light, such as light-emitting diodes that make up displays, are present in many different kinds of electronic equipment. In all such devices, an organic active layer is sandwiched between two electrical contact layers. At least one of the electrical contact layers is light transmitting so that light can pass through the electrical contact layer. The organic active layer emits light through the light-transmitting electrical contact layer upon application of a voltage across the electrical contact layers.
Due to rapid progress in the development of organic light-emitting materials, devices based on these materials, called PLEDs and OLEDs (polymer and small-molecule organic light-emitting diodes), are entering the display market.
A very promising alternative to PLED/OLED, particularly for large-area lighting applications, is the light-emitting electrochemical cell (LEEC) [1]. A LEEC does not need a low-workfunction metal electrode and thicker organic active layers can be used, while keeping the operating voltage low. The operating mechanism is based on the presence of mobile ions.
FIG. 1 schematically shows the operating mechanism of a LEEC; the top pictures are cross sections, and the bottom pictures are energy band diagrams. (a) shows the relative positions of the energy levels when the layers are not in contact: the Fermi levels of the electrodes are not matched with the HOMO and LUMO levels of the electro luminescent layer. The ions in that layer reside in pairs. (b) shows the situation when a voltage is applied high enough to overcome the band gap of the electro luminescent layer: the ions have moved to opposite electrodes so that strong voltage drops are created, making charge carrier injection and thus electro luminescence possible.
Thus, upon application of a voltage, the cations and anions move towards the cathode and anode respectively, leading to large electric fields at the electrode interfaces. The ion distribution formed facilitates injection of electrons and holes at the cathode and the anode respectively, thus allowing transport and recombination of the charge carriers, which results in emission of a photon.
For lighting applications the generation of white light is essential. In case of organic light emitting devices, this can be obtained by e.g. combining blue and orange/yellow emission or blue, green and red emission. The orange/yellow, green and red emission can be obtained by electro luminescence, or by photoluminescence upon absorbing part of the emitted blue light. In all cases, the generation of blue light by electro luminescence is necessary.
The highest efficiencies of light generation are achieved by using triplet emitters, in particular Ir-complexes. These can be made to emit light of any colour by proper substitution of the ligands and proper charge of the complex. For instance, [Ir(ppy)2(bpy)]+(PF6−) emits yellow light.
The photophysical and photochemical properties of d6 metal complexes such as ruthenium (II), osmium (II), rhenium (I), rhodium (III) and iridium (III) have been thoroughly investigated during the last two decades. The fundamental thrust behind these studies is to understand the energy and electron transfer processes in the excited state and to apply this knowledge to potential practical applications, such as solar energy conversion, organic light-emitting diodes, electro luminescence and in sensors. The main requirements for organic light-emitting devices are that the complexes should exhibit very high phosphorescence quantum efficiencies and sharp emission spectra in the visible region, preferably with the maxima around 440 nm (blue), 530 nm (green) and 640 nm (red). This is of importance for display (wide colour gamut) as well as lighting (high colour rendering index) applications. Several groups have used extensively iridium (III) based complexes in light-emitting devices and obtained up to 12.3% external quantum efficiencies [5].
The origin of emission in iridium complexes containing the 2-phenylpyridine ligand is the charge transfer excited states decay through radiative pathways, which are known to exhibit high quantum yields due to mixing the singlet and the triplet excited states via spin-orbit coupling. Nevertheless, the majority of charged iridium (III) complexes known to date largely remained as green or yellow emitters and pure blue and red emitting complexes are scarce [6].
Blue electro luminescence has been obtained by fluorine substitution of the ligands of Ir complexes [2], [3], [4]. Fluorine substitution is also described in US 2005/0037233. One disadvantage is that all fluorine-substituted blue-emitting Ir complexes known are neutral, which is a disadvantage for LEECs. The metal complex emitters for LEECs should preferably be charged since then they also provide the ions needed to enable charge injection. Also for neutral Ir complexes it remains a challenge to shift the emission spectrum further into the blue and red. Further, for lighting applications, it may be advantageous to be able to tune the emission wavelength to other colours, e.g. yellow or orange as described above. Thus, there is a continuing need for electro luminescent compounds.